The present invention relates to a motor and more particularly to circuitry for driving a motor by PWM (Pulse Width Modulation) control while reducing noise ascribable to the rotation of the motor with a simple configuration, and a motor driving method therefor.
For the drive of a brushless motor, there is an increasing demand for PWM control that implements efficient rotation. There is also required a method capable of driving the motor while reducing noise ascribable to the rotation of the motor. However, conventional PWM type of motor drive circuitry lacking a measure against noise has the following problems (1) through (3) left unsolved.
(1) At the time of phase switching, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are sharply turned on by hard switching, causing current to sharply change. The resulting pulses generate noise inductively coupled to the floating coil or a plurality of coils of a motor.
(2) The level of noise ascribable to rotation differs from one motor to another motor, so that a noise reduction time must be finely control led motor by motor. Such fine control, however, is not easy to execute because the noise reduction time cannot be controlled by a capacitor or similar part that is easy to replace.
(3) The circuitry is scaled up because it is sophisticated and uses many amplifiers whose size is likely to increase.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-126381 and 11-235079 and Japanese Patent Nos. 2,721,081 and 3,015,588.
It is therefore an object of the present invention to solve the problems (1) through (3) stated above.
It is another object of the present invention to maintain, when driving a brushless motor by PWM control, a noise reduction time constant without regard to the rotation speed of the motor for thereby preventing efficiency from noticeably falling at high rotation speeds, while saving energy.
Circuitry for driving a motor by PWM control of the present invention includes switching devices respectively associated with drive coils, which are included in the motor and assigned to a particular phase each. A frequency oscillator generates a triangular wave. A first comparator generates a pulse sequence by comparing the triangular wave and a voltage for PWM oscillation frequency modulation. A position sensing circuit senses the angular positions of the motor. A pulse generating circuit counts a preselected number of pulses, which are included in the pulse sequence, by using each of the positive-going and negative-going edges of position signals output from the position sensing circuit as a trigger to thereby generate pulse signals. A voltage adjusting circuit converts a voltage for duty modulation to a plurality of voltage levels. A voltage selector selects one of the voltage levels in accordance with the pulse signals output from the pulse generating circuit, and outputs the voltage level selected as a duty modulation voltage. A second comparator outputs a first duty signal by comparing the voltage for duty modulation and the triangular wave. A third comparator outputs a second duty signal by comparing the duty modulation voltage output from the voltage adjusting means and the triangular wave. An exciting pulse generating circuit generates, based on the position signals output from the position sensing circuit and the first and second duty signals, a gate signal for switching control for a preselected period of time at each time of phase switching. The gate signal includes a signal for reducing the noise of the motor.
A motor driving method for the above circuitry is also disclosed.